1. Field of the Invention
The invention relates to spectrographs, particularly spectrographs used to cover large spectral intervals.
2. Description of the Prior Art
A spectrograph is designed to form a spectrally dispersed image of an entrance aperture on a flat, accessible focal surface, where the entire image of the spectrum may be recorded simultaneously with a photographic plate or a high-efficiency array of solid-state detectors. Because this problem has been central to many branches of scientific research for decades, hundreds of designs exist to perform the same function.
A critical feature of the design of any spectrograph is the dispersing element. Two broad classes of spectrographs may be defined according to the type of dispersing element employed: a prism or a diffraction grating. Because diffraction gratings can cover no more than a one octave spectral interval (a factor of two in wavelength interval) without the problem of overlapping orders, diffraction gratings are not suited to an application that requires that a large spectral interval be covered simultaneously without moving any of the optical elements or the detector in the spectrograph. In contrast, prism spectrographs are particularly suitable for surveying unknown spectra, where large spectral intervals need to be covered efficiently, without ambiguity, but at a relatively low resolving power.
One definition of spectrograph resolving power that is useful in the case of an array detector is the ratio of the central wavelength seen by a given detector to the change in wavelength which causes the signal on that detector to fall to one half of its value at the central wavelength (Full Width at Half Maximum).
Prism spectrographs can be further distinguished based on the nature of the elements used to perform two necessary functions: dispersing and reimaging the incident energy. Configurations in which one or two elements perform both functions are rare, but usually result in compact instruments because of the reduced number of optical elements. Fewer parts usually means improved mechanical stability and optical transmission efficiency. Fewer parts also means reduced size and weight. These features are particularly important in applications involving cryogenic instruments or spacecraft instruments.
The prior art discloses folded prism spectrographs in which aplanatic refractions are used to minimize the optical aberrations of the system. Aplanatic refractions are refractions that introduce no spherical aberration or coma. For example, U.S. Pat. No. 2,866,374, issued to Lewis and Thomas, discloses a "Monochromator" and Wilson, U.S. Pat. No. 3,625,615, discloses "A Device for Spectral Dispersion of Light Employing a Predispersion Prism and a Grating Monochromator". Both of these devices provide examples of folded prism spectrographs with aplanatic surfaces in which the functions of dispersing and reimaging are combined into one or two elements.
The previous work by Lewis and Thomas and by Wilson concentrated on monochromators, a very specific and basic form of spectrograph. Monochromators are intended to image only one wavelength at a time. A mechanical motion is used to scan an extended spectrum past a single detector. Because only one wavelength at a time needs to be in sharp focus, chromatic variations in image quality can also be compensated for with small adjustments in the Position and orientation of the optical elements. A flat focal plane is unnecessary. Finally, only a relatively small separation is required between entrance and exit apertures, sufficient only to permit light to be inserted and extracted from the device.
However, monochromators are too limited to perform well as spectrographs. A spectrograph must form good images simultaneously at all operating wavelengths, not just a single wavelength. Moreover, the surface containing these images should be flat so that solid state detector arrays can be used to record the spectrum. This surface should also be as nearly as possible perpendicular to rays incident on the focus in order to reduce reflection losses at the detector and to ensure that light falling on one part of array is not transmitted to another via multiple internal reflections. Normal incidence of the light also reduces problems with anamorphic magnification. Anamorphic magnification is a difference in focal length along and perpendicular to the direction of dispersion and can result in non-optimal use of the elements in a detector array. Finally, the physical dimensions of the detector array in the spectrograph and its mounting typically require a significant separation between the entrance aperture and the dispersed images.
Therefore, the principal object of the present invention is to provide a spectrograph capable of simultaneously surveying more than a one octave spectral interval at a low (20-100) resolving power. Another object of the present invention is to provide a spectrograph capable of producing good image quality over a broad range of operating wavelengths simultaneously. Another object is a spectrograph with a flat focal surface nearly normal to the incident light and well separated from the entrance aperture. Another object of the Present invention is to provide a spectrograph with high optical throughput, with an f-number faster than f/3. Yet another object is to provide a spectrograph consisting of elements with easy-to-fabricate spherical surfaces. Another object is to provide a spectrograph in which all detectors in the spectrograph view an object through the same aperture, thereby preventing variations in source position and intensity with time from introducing any ambiguities into the spectra.